For the 640XL you xan use a "memory expansion card" but it has to be a micro SD card. The size is up to you but currently the biggest size ,for a "reasonable" price, is 128 GB. I would recommend a class 10 from a "know" brand and trustworthy "shop" so Amazon and Newegg are both "good" shops. Now SD cards also have UHS rattings but for your phone they do not matter because your phone can not "use" UHS it only can use the Class ratting. The only advantage of an UHS is that they are faster if you put them in your card reader from your PC. There are "regular" discount actions for Micro SD cards so if you are not in a hurry you can wait for a discount action.

For a 640xl u need a "micro SD" type with storages starting from 1 gigabyte all the way up to 128 gigabytes. For a heavy media consumer "music downloads, movies, podcasts and apps" 64 gigabytes is more than enough. As for the class of the card well it doesn't really matter if ur gonna get anything between 4 and 10 "transfere speed differs incrementally by buying a higher class card". As for manufacturers I recommend one of the following. Kingston, SanDisk, corsair or Samsung.